1. Field of the Invention
The present invention relates to well logging. In particular, the present invention is an apparatus and method for imaging of subsurface formations using electrical methods.
2. Description of the Related Art
U.S. Pat. No. 4,468,623, issued to Gianzero, discloses tool mounted pads, each with a plurality of small measure electrodes from which individually measurable survey currents are injected toward the wall of the borehole. The measure electrodes are arranged in an array in which the measure electrodes are so placed at intervals along at least a circumferential direction (about the borehole axis) as to inject survey currents into the borehole wall segments which overlap with each other to a predetermined extent as the tool is moved along the borehole. The measure electrodes are made small to enable a detailed electrical investigation over a circumferentially contiguous segment of the borehole so as to obtain indications of the stratigraphy of the formation near the borehole wall as well as fractures and their orientations. In one technique, a spatially closed loop array of measure electrodes is provided around a central electrode with the array used to detect the spatial pattern of electrical energy injected by the central electrode. In another embodiment, a linear array of measure electrodes is provided to inject a flow of current into the formation over a circumferentially effectively contiguous segment of the borehole. Discrete portions of the flow of current are separably measurable so as to obtain a plurality of survey signals representative of the current density from the array and from which a detailed electrical picture of a circumferentially continuous segment of the borehole wall can be derived as the tool is moved along the borehole. In another form of an array of measure electrodes, they are arranged in a closed loop, such as a circle, to enable direct measurements of orientations of resistivity of anomalies.
The device of Gianzero '623, is primarily designed for highly conductive mud. In oil based muds, (OBM) the currents flowing from the electrodes depend upon good contact between the electrode and the borehole wall. If the borehole wall is irregular, the contact and the current from the electrodes is irregular, resulting in inaccurate imaging of the borehole. Finally, being a wireline tool, a plurality of contact pads disposed around the logging tool must be used to get a 360° scan of the borehole.
OBM increase drilling efficiency due to better lubrication of drill bits. In addition, increasing number of present day exploration prospects lie beneath salt layers that are water soluble, necessitating the use of OBM for drilling
U.S. Pat. No. 3,973,181 to Calvert teaches a method and apparatus for investigating earth formations traversed by the borehole containing an electrically nonconductive fluid in which a system of pad-mounted electrodes are arranged laterally around a well tool. A high frequency oscillator is coupled to the electrodes through a selectively variable capacitor. As the apparatus is passed through the borehole, the high frequency current produced by the oscillator capacitively couples the electrodes to the formation and provides a measure of the electrical conductivity or resistivity of the earth formation.
U.S. Pat. No. 4,052,662 to Rau et al discloses a wireline resistivity measuring device that preferably operates in the frequency range of 300 MHz to 300 GHz. In order to make accurate phase measurements at such high frequencies, a heterodyning principle is used with the received signals being mixed with an oscillator that has an output frequency that differs from the frequency of the transmitted signal: the difference may be 100 kHz or so. The addition of a mixer and the secondary oscillator, adds greatly to the complexity of the hardware. Such heterodyning has commonly been used in high frequency resistivity measuring devices.
U.S. Pat. No. 5,339,036 to Clark et al teaches a MWD resistivity device in which button electrodes are mounted on a stabilizer blade. The device of Clark is primarily designed for use with WBM and requires that the button electrodes be in close contact with the formation. In a MWD tool, this results in rapid wearing away of the electrodes, necessitating frequent replacement.
U.S. patent application Ser. No. 09/836,980 of Evans et al. having the same assignee as the present application and the contents of which are fully incorporated herein by reference, teaches an apparatus and method based on the use of high frequency measurements for injecting measure currents into the formation. One embodiment of the device and method taught in the Evans '980 application uses a carrier frequency that is sufficiently high to capacitively couple the measure electrodes to the formation through the nonconducting mud. By modulating the carrier current at a frequency used in prior art resistivity imaging devices, it is possible to get measurements of formation resistivity with relatively minor changes in the hardware of prior art resistivity devices, the main modification being a modulator for producing the measure currents demodulator for demodulation of the measure currents.
U.S. patent application Ser. No. 10/090,374, of Evans, having the same assignee as the present application and the contents of which are fully incorporated herein by reference, discloses a method for obtaining a resistivity image of an earth formation penetrated by the borehole. The apparatus includes at least one measure electrode that injects a measure current into the formation. Due to the high frequency of the current, an electrical circuit is complete when the borehole is filled with a non-conductive fluid through a capacitive gap between the electrode and the formation. A guard potential is provided to maintain focusing of the current. The modulation of the measure current and the demodulation of the output of the current measuring circuit helps reduce the cross-talk between them. Amplitude modulation reverse amplitude modulation, frequency modulation or phase modulation may be used.
U.S. Pat. No. 6,173,793 B1, issued to Thompson et al., is an apparatus and method for obtaining information about a formation using sensors on a substantially non-rotating pads attached to a rotating housing that is part of the drilling assembly. The pads make contact with the formation. The sensors may be density, NMR, resistivity, sonic, or electromagnetic. The NMR sensors may use a static magnetic field that can be either radial or longitudinal in direction. The resistivity sensors may involve direct measurement of leakage current or may rely on induction methods. In an alternate arrangement, the sensors rotate with the drill bit. A downhole microprocessor analyzes the data to improve signal-to-noise ratio and to reduce redundancy in the acquired data. Depth information may be telemetered from an uphole controller to facilitate the process.
A need exists for obtaining electrical measurements in a robust MWD logging device for use with OBM and WBM that obtains higher resolution and azimuthal recording capabilities. Such a device should preferably be simple. The present invention fulfills this need.